The British far-right activist Tommy Robinson is receiving financial, political and moral support from a broad array of non-British groups and individuals, including US thinktanks, rightwing Australians and Russian trolls, a Guardian investigation has discovered.

Robinson, an anti-Islam campaigner who is leading a “Brexit betrayal” march in London on Sunday, has received funding from a US tech billionaire and a thinktank based in Philadelphia.

Two other US thinktanks, part-funded by some of the biggest names in rightwing funding, have published a succession of articles in support of Robinson, who has become a cause célèbre among the American far right since he was jailed in May for two months.

His imprisonment on contempt of court charges prompted a vigorous international Twitter campaign, with 2.2m tweets being posted using the hashtag #freetommy between May and October.

An analysis conducted for the Guardian by the London-based Institute for Strategic Dialogue found that more than 40% of the tweets came from the US, 30% from the UK and other significant volumes from Canada, the Netherlands and nine other countries.

A separate study of about 600 Twitter accounts, believed to be directly tied to the Russian government or closely aligned with its propaganda, found significant numbers had tweeted prolifically in Robinson’s defence.

On Facebook, Robinson has more than 1 million followers from at least a dozen countries outside the UK, including the US, Australia, Sweden and Norway.

Robinson, whose real name is Stephen Yaxley-Lennon, has been using Facebook donation tools designed for charities to raise funds for his activism for several months.

He says he has raised several hundred thousand pounds via online donations, some of which were solicited via the Facebook donate button. Robinson has said he plans to use the money to launch a European version of the rightwing conspiracy website Infowars, and to sue the British government over his prison treatment.

But the tool is meant for charities alone. When the Guardian alerted Facebook to this, the social media company switched off the function within hours.

The Guardian looked into Robinson’s global support after he was jailed for filming outside a rape trial involving defendants of mainly Pakistani heritage at Leeds crown court. He was released on 1 August after the court of appeal ordered that he should be retried. The attorney general is deciding whether to proceed with a retrial.

The investigation has established that:

A Philadelphia-based thinktank, the Middle East Forum (MEF), acknowledges it has spent about $60,000 (£47,000) on Robinson’s legal fees and demonstrations staged in London earlier this year. A senior MEF executive has been closely involved in preparations for this weekend’s march, though the thinktank said she was there in a personal capacity.

A US tech billionaire, Robert Shillman, financed a fellowship that helped pay for Robinson to be employed in 2017 by a rightwing Canadian media website, the Rebel Media, on a salary of about £5,000 a month.

A small Australian rightwing group, Australian Liberty Alliance, says it has helped fund Robinson, but did not disclose how much.

A New York City-based thinktank, the Gatestone Institute, has published a succession of articles supporting Robinson’s cause.

The David Horowitz Freedom Center (DHFC), a California-based thinktank that describes itself as a “school for political warfare”, has published a series of pieces defending Robinson, and has lobbied for him to address US politicians.

Horowitz, the co-founder of the DHFC, told the Guardian in an email: “Tommy Robinson is a courageous Englishman who has risked his life to expose the rape epidemic of young girls conducted by Muslim gangs and covered up by your shameful government.”

MEF, Gatestone and the DHFC are well funded by influential rightwing donors, according to tax returns scrutinised by the Guardian. In 2014-16, the returns show they received a total of almost $5m from several millionaire donors.

The DHFC received $1,638,290 from five wealthy benefactors, one of whom is believed to be among the biggest-ever donors to the Republican party.

Gatestone has received more than $2m in donations, including $250,000 from the Mercer Family Foundation, which is funded by Donald Trump’s top donor, Robert Mercer, and run by the billionaire’s daughter Rebekah.

All three thinktanks have been repeatedly accused of stoking anti-Islam sentiment in the west and spreading false information about Muslim refugees in Europe. But all three have consistently denied being anti-Islam.

“Radical Islam is the problem and moderate Islam is the solution,” the MEF president, Daniel Pipes, said in an email. “MEF fights for the right to discuss Islam and related issues in free, robust, open and public debate.”

Pipes added that he believed Robinson had been prosecuted for his views and not his actions outside the courthouse.

He said: “In May 2018, in the course of five hours, he was arrested, tried, convicted, sentenced to 13 months’ prison, and jailed; that sounds more like a banana republic than the home of the Magna Carta.”

Rosenwald and the Gatestone Institute have strongly denied they are anti-Islam. In a 5,000-word article in May, the institute said “far from being anti-Muslim” it was “pro-Muslim” and that it did not want to see “Muslims deprived of freedom of speech, flogged or stoned to death for supposed adultery”.

A spokesman said: “Gatestone is a free speech platform and publishes hundreds of online articles a year expressing a varied range of views, including articles by Muslims, and does not endorse the comment of all its contributors.”

Robinson, Shillman and the Mercers did not respond to detailed requests for comment.

The support from prominent and well-financed groups undermines Robinson’s self-styled image of a far-right populist underdog whose anti-Islam agenda is being silenced by the British establishment.

Robinson was recently appointed an official adviser to Ukip, which is backing his pro-Brexit rally on Sunday. Ukip’s embrace of him has caused a rupture in the party and prompted two former leaders, Nigel Farage and Paul Nuttall, and hundreds of members to leave.

Robinson founded the English Defence League, a far-right Islamophobic group, in 2009. It has since fractured and declined. He frequently complains of being smeared as a racist, insisting he does not care about skin colour and that his objection is to Islamist political ideology rather than people.

Tommy Robinson: from local loud mouth to international far right poster boy

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However, he has been filmed saying things like: “Somalis are backward barbarians”; British Muslims are “enemy combatants who want to kill you, maim you and destroy you”; and refugees are “raping their way through the country”.

The news of his imprisonment on 25 May generated a surge of pro-Robinson tweets. An analysis of 2.2m #freetommy tweets between May and October showed 42% came from the US, according to research by the Institute for Strategic Dialogue.

A second analysis, by the Alliance for Securing Democracy (ASD) thinktank, discovered social media backing from a cluster of 600 accounts it identified as being aligned with the Kremlin. Pro-Robinson tweets accounted for three of their top five most-used hashtags on 27 May,and most pointed users to articles on the rightwing websites InfoWars, Breitbart and Voice of Europe, according to the researchers.

“The clustered focus on the Tommy Robinson case in late May suggests that Russian-linked accounts saw his arrest as a clear opportunity to amplify political divisions both in the UK and abroad,” said Bret Schafer, a social media analyst at the US-based ASD.

Fiyaz Mughal, the founder of Tell Mama, which records anti-Muslim hate crimes, described the US and Russian support for Robinson as foreign interventionism. He said: “It should alarm anyone in this country who values the democratic principles on which our country are founded.”